As the world's need for communication capacity continues to increase, the use of optical signals to transfer large amounts of information has become increasingly favored over other schemes such as those using twisted copper wires, coaxial cables, or microwave links. Optical communication systems use optical signals to carry information at high rates over an optical path such as an optical fiber. Optical fiber communication systems are generally immune to electromagnetic interference effects, unlike the other schemes listed above. Furthermore, the silica glass fibers used in fiber optic communication systems are lightweight, comparatively low cost, and are able to carry tens, hundreds, and even thousands of gigabits per second across large distances.
Indeed, one 1998 experiment was reported in which a single optical fiber successfully transported 2,640 gigabits per second of information across a 120 km distance; see Dutton, Understanding Optical Communications (Prentice Hall 1998), which is incorporated by reference herein, at p. 6. According to Dutton, supra, this single optical fiber was transporting enough data to simultaneously carry the maximum number of telephone calls in existence on Earth at any particular moment in time (about 30 million calls in 1998). Most practical commercial systems operate at lower data rates, of course, although progress continues in increasing the data rate of practical optical communication systems.
Today's long-distance fiber optic communication systems generally use single-mode optical fibers to transport light at wavelengths between 1530 nm and 1570 nm in the infrared spectrum. As known in the art, the 1530-1570 nm range is the operational range of Erbium-doped fiber amplifiers (EDFAs), which are currently used in practical long-distance fiber optic communications links. Also, the 1530-1570 nm range is where current single-mode fibers have relatively low attenuation (about 0.26 dB/km). Accordingly, it is desirable to carry as much information as possible in this 1530-1570 nm wavelength band. However, known current electro-optical modulators are only capable of modulating light at a rate of about 10 GHz, corresponding to a wavelength spread of only about 0.16 nm. To efficiently use the entire 1530-1570 nm spectrum, wavelength-division multiplexing (WDM) is used, wherein a plurality of light beams are separately modulated and are then optically combined onto a single optical fiber. To maximize the use of available spectrum, as many channels as possible are multiplexed together into the 1530-1570 nm band in what are often referred to as dense wavelength-division multiplexing (DWDM) systems.
FIG. 1 shows a portion of a spectrum 102 of a typical DWDM signal, in which forty channels are spaced 50 GHz apart (0.4 nm apart) and multiplexed onto a single optical fiber. The width of each channel depends on the specific modulation scheme and modulation rate for that channel, but the channels are shown generally in FIG. 1 as having a width of about 20 GHz (0.16 nm). For increased efficiency and network throughput, it is desirable to increase the number of channels multiplexed onto the single fiber, while also retaining the ability to effectively filter out (“drop”) or add in (“add”) any given channel on the fiber at a given point. The ability to add and drop individual channels from a data stream is, of course, central to the construction of DWDM-based optical communication networks. In FIG. 1, for example, it is necessary to be able to filter out channel 104 without crosstalk effects from neighboring channels 106 and 108.
As international standards continue to reduce DWDM channel spacings, conventional dichroic filters used in add/drop devices have not been able to keep up in terms of the narrowness of their passband, with the known best dichroic filters only achieving effective channel separation for channel spacings of about 100 GHz or greater. To address this problem, optical interleavers have been introduced that are capable of operating on a DWDM signal carrying channels at wavelength (λ1, λ2, λ3, λ4, λ5, λ6, . . . ) to produce two output signals carrying channels at (λ1, λ3, λ5, . . . ), and (λ2, λ4, λ6, . . . ), respectively. Because the channel spacings are now doubled, it is easier for conventional dichroic filters to operate on the signal to drop and/or add the channels of interest.
An idealized optical interleaver is shown as element 110 in FIG. 1, comprising an input port 112 for receiving the initial DWDM signal 102 and two output ports 114 and 116 for generating the output signals 118 and 120, respectively. The idealized filter function from input port 112 to output port 114 is represented by Ieven while the idealized filter function from input port 112 to output port 116 is represented by Iodd. As indicated in FIG. 1, the ideal spectral response of an optical interleaver is a perfectly square or box-like response. As described infra, practical optical interleavers will generally have passband characteristics with narrower tops and wider bottoms than the ideal.
Unless otherwise indicated, it is to be appreciated that the interleavers described herein are reciprocal devices, with the term “interleaver” representing a device that will perform “de-interleaving” when light signals are passed through in the reverse direction in accordance with optical reciprocity principles. Thus, for example, the interleaver 110 of FIG. 1 actually works as a “de-interleaver” when provided with a signal at port 112, providing the de-interleaved outputs at ports 114 and 116, and will operate as an “interleaver” when provided, for example, with the appropriate component signals at ports 112 and 116 providing the interleaved output at port 114.
FIG. 2 shows a generic spectral response characteristic 202 of one output port of a practical optical interleaver, which differs from the idealized optical interleaver 110 in that the passbands are not perfectly square. Different metrics are used to evaluate the squareness of the interleaver passbands, one such set being shown in FIG. 2. The metric FSRout is the free spectral range of the output, i e., the distance between transmission peaks, which is usually twice the input channel spacing Δf. Whereas the spectral response characteristic 202 may represent the “even” output channels from a first output port, there is complementary spectral characteristic (not shown) for the “odd” channels which is similar to the curve 202 shifted by Δf. The metrics W0.1 dB, W0.5 dB, W3 dB, W20 dB, and W30 dB represent the width of the spectral curve around one of the passbands of the optical interleaver at spectral power attenuations of 0.1 dB, 0.5 dB, 3 dB, 20 dB, and 30 dB, respectively, as shown in FIG. 2, and may be represented in distance (nm) or corresponding frequency (GHz) units.
For an ideal interleaver having a perfect box-like response, each of the values W0.1 dB, W0.5 dB, W3 dB, W20 dB, and W30 dB would be equal to Δf. In general, the closer that all of these values are to Δf, the better the performance of the optical interleaver as there is reduced crosstalk between channels and less distortion in the passed signals. It is particularly desirable to make an optical interleaver having both W0.1 dB and W0.5 dB as wide as possible to create a flat passband for reduced distortion. It is also desirable to make an optical interleaver having both W20 dB and W30 dB as narrow as possible to reduce crosstalk between a passed channel (e.g., λ2) and its immediately neighboring channels (e.g., λ1 and λ3). For comparative purposes, the values of the width metrics W0.1 dB, W0.5 dB, etc., may be normalized by the input channel spacing Δf and/or the free spectral range FSRout, in which case their optimal normalized values would be 1.0 and 0.5, respectively. Unless otherwise indicated, in the present disclosure the width metrics W0.1 dB, W0.5 dB, etc. will be normalized by the input channel spacing Δf.
FIG. 3 shows an optical interleaver 300 according to the prior art, in the form of a classical Michelson interferometer. Optical interleaver 300 comprises a beamsplitter 302 for dividing an incoming beam, received at an input port 301 and having WDM channels at (λ1, λ2, λ3, λ4, λ5, λ6, . . . ), into a first beam 304 and a second beam 306, respectively, these beams being separated by 90 degrees as shown in FIG. 3. Optical interleaver 300 further comprises a first mirror 308 for receiving and reflecting the first beam 304 back to the beamsplitter 302, and a second mirror 310 for receiving and reflecting the second beam 306 back to the beamsplitter 302, to the same point on the beamsplitter 302 as the first reflected beam. The beamsplitter 302, which also operates as an optical combining device, causes the first and second beams 304 and 306 to interferometrically combine to produce a first output 312 and a second output 314. Distances L1 and L2 separate the beamsplitter 302 from the first mirror 308 and the second mirror 310, respectively. As known in the art, taking into account the total optical path ΣLiηi traversed by each of the first beam 304 and second beam 306, including glass portions or other material in which i is greater than 1, the dimensions L1 and L2 may be selected such that, for the first output 312, constructive interference occurs for the odd wavelengths (λ1, λ3, λ5, . . . ) and destructive interference occurs for the even wavelengths (λ2, λ4, λ6, . . . ). Likewise, using these dimensions, for the second output 314 there will be constructive interference for the even wavelengths (λ2, λ4, λ6, . . . ) and destructive interference for the odd wavelengths (λ1, λ3, λ5, . . . ). The second output 314 containing the even wavelengths may then be separated from the incoming beam using a circulator (not shown).
The optical interleaver 300, however, contains several shortcomings which would limit its use as a practical interleaving device. First, it can be readily shown that the transmission response of either output, for example the even output 314, is merely a sinusoidal function having maxima at (λ2, λ4, λ6, . . . ) and minima at (λ1, λ3, λ5, . . . ). In particular, the transmission characteristic metrics defined supra would be approximately W0.1 dB=0.19Δf, W0.5 dB=0.43Δf, W3 dB=1.00Δf, W20 dB=1.87Δf, and W30 dB=1.96Δf. These values would lead to large amounts of crosstalk and signal distortion.
Moreover, the optical interleaver 300 is polarization-sensitive, the output being degraded if the incident light is partially or totally polarized. This is due to the fact that the beamsplitter 302 as configured in FIG. 3 will not perform a precise 50/50 split of incident light that is partially or totally polarized. In turn, it is this precise 50/50 split that is depended upon for proper constructive and destructive interference of the reflected beams. It has been found that even when unpolarized light is introduced into a fiber in a real-world optical fiber communication system, unwanted polarizations can arise due to many different factors, such as excessive twists or loops in the optical fiber. It is therefore required that optical interleavers be insensitive to polarizations in the incident light beam.
Finally, the optical interleaver 300 is not thermally robust, with the output degrading upon substantial temperature fluctuations sufficient to change the index of refraction of the glass (or other optical material) portions of the optical paths of beams 304 and 306. In particular, when the temperature rises and the refractive index η of the glass portions increase, the beam 306 will be retarded by a greater amount than beam 304 because it travels through more glass than beam 304. This, in turn, disturbs the phase relationships required for proper constructive and destructive interference, degrading the operating characteristics of the optical interleaver.
Some proposals have been made for providing split-beam interferometer-based optical interleavers that provide better performance than the classical Michelson interferometer, including that discussed in WO 00/48055 (“the '055 reference”), which is incorporated by reference herein. The '055 reference discusses an optical interleaver comprising a “separator” having a surface with a 50% reflective coating oriented at what is apparently 45 degrees with respect to the incoming beam ('055 reference, FIG. 4) that splits an incoming beam into two component beams at an apparent 90 degree angle with respect to each other. The two beams are then directed in parallel to a “nonlinear split beam interferometer,” with the reflected beams being recombined at the separator to produce the interleaved outputs. The “nonlinear split beam interferometer” ('055 reference, FIG. 5) comprises a cavity having a front surface of 18.5% reflectivity, a rear surface of 100% reflectivity, and a “wavelength tuning element” interposed therebetween. The cavity also comprises a “180 degree phase bias element” interposed between the reflective surfaces, along with a “90 degree phase bias element” outside the cavity that is encountered by only one of the beams.
The interleaver of the '0.55 reference, however, has several disadvantages. First, although the '0.55 reference describes the 50% reflective coating on the separator to be “polarization insensitive,” it is not apparent what such material is and/or whether it can be physically realized. If not, the interleaver of the '0.55 reference would be highly polarization sensitive. It would be desirable to provide an optical interleaver having reduced polarization sensitivity that does not depend on the existence of a “polarization insensitive” reflective coating.
Second, the interleaver of the '0.55 reference will be sensitive to thermal variations. As shown in FIG. 4 therein, each split beam of the '0.55 reference encounters an identical physical distance between the separator and the cavity, with the optical path difference being created by virtue of placing glass or other solid material (the “phase bias elements”) in the path of one of the beams. However, it is well known that most materials including glass will have a refractive index η that changes with temperature, while the refractive index of air or vacuum is comparatively insensitive to thermal variations. Accordingly, the amount of optical path difference between the split beams will vary with temperature, making the interleaver of the '0.55 reference thermally sensitive and/or thermally unstable.
Third, the interleaver of the '0.55 reference comprises a cavity designed such that the split beams encounter surfaces of identical reflectivities (i.e., both beams encounter front surfaces of 18% reflectivity and rear surfaces of 100% reflectivity). While some performance improvement over the classical Michelson interferometer may be realized (e.g., a W20 dB of about 1.50 of the input channel spacing versus 1.87 for the classical Michelson interferometer, see '0.55 reference, FIG. 3, plot 340), it would be desirable to provide an optical interleaver having a spectral characteristic that is closer to the ideal box-like response.
Another optical device that alters a classical Michelson interferometer is discussed in EP0933657(A2) by Dingel et al (“the '657 reference”), which is incorporated by reference herein. The '657 reference discusses a “Michelson-Gires-Tournois Interferometer (MGTI)” in which one of the reflecting mirrors of a classical Michelson interferometer of FIG. 3 is replaced by a Gires-Tournois resonator having a partially reflective front mirror and a fully reflecting back mirror. However, for reasons similar to those described supra with respect to the classical Michelson interferometer, the device of the '657 reference is sensitive to polarization of the input light. Also, the '657 reference discusses a device in which the free spectral range of the output is much greater (e.g., 30.8 nm) than that required for use as a WDM interleaver, and would fit only one passed channel in the 1530-1570 nm operational range of Erbium-doped fiber amplifiers (EDFAs). Moreover, it would be desirable to provide an optical interleaver having a spectral characteristic that is still closer to an ideal box-like response.
Accordingly, it would be desirable to provide an optical interleaver for use in optical communications systems that provides an output characteristic that is close to an ideal box-like response.
It would be further desirable to provide an optical interleaver that has reduced sensitivity to polarization of the incident light.
It would be still further desirable to provide an optical interleaver having increased thermal stability.